The present invention relates to electrical circuits and more particularly to direct current (DC) to direct current (DC) power conversion and regulation.
There is an ever increasing demand for power conversion and regulation circuitry to operate with increased efficiency and reduced power to accommodate for the continuous reduction in size of electronic portable devices. Many times these devices are battery powered, and it is desirable to utilize as little power as possible to operate these devices, so that the battery life is extended. Therefore, the prior 5-volt industry standard has decreased to a 3.3 volt industry standard, which may soon be replaced by an even lower standard. As the regulated voltage levels decrease, the effect that traditional components (e.g., diodes) have on efficiency increases. Additionally, a secondary or auxiliary voltage (e.g., 5 volts) is often desired to operate components having a different voltage requirement than the components utilizing the primary voltage.
Voltage regulators have been implemented as an efficient mechanism for providing a regulated output in power supplies. One such type of regulator is known as a switching regulator or switching power supply, which controls the flow of power to a load by controlling the on and off duty-cycle of one or more power switches coupled to the load. Many different classes of switching regulators exist today. One class of switching regulators is known as a transformer-coupled switching regulator, such as a coupled-inductor buck converter. A conventional coupled-inductor buck converter provides multiple regulated outputs by generating a primary output using a buck converter and adding one or more auxiliary windings to the output inductor, which operate as coupled inductors for secondary or auxiliary outputs.
FIG. 1 illustrates a conventional switching regulator 10 (e.g., a synchronous buck converter). The switching regulator 10 includes a control circuit 12 that is operative to control the duty cycle of pulses provided to a first power switch 14 and a second power switch 16. In the illustration of FIG. I, the first power switch 14 and the second power switch 16 are N-type MOSFET devices coupled in series. The first power switch 14 is coupled to an input voltage (VIN) at its drain terminal and a node 18 at its source terminal. The second power switch 16 is coupled to the node 18 at its drain terminal and to ground at its source terminal. The node 18 is coupled to a primary inductor 24, which provides the energy to charge a capacitor 26. The control circuit 12 switches a control pulse between the first power switch 14 and the second power switch 16 in opposing states, causing the first power switch 14 to turn xe2x80x9cONxe2x80x9d and the second power switch 16 to turn xe2x80x9cOFFxe2x80x9d. The switching of the first and second power switches provides an input pulse signal, similar to a square wave, that toggles between VIN and ground at the node 18 and the primary inductor 24.
Energy builds up on the primary inductor 24 when the first power switch 14 is xe2x80x9cONxe2x80x9d, which is transferred to charge the capacitor 26 to an output voltage VOUT1. A feedback signal from the output voltage VOUT1 of the capacitor 26 is fed back to the control circuit 12. The control circuit 12 utilizes the feedback signal to continuously adjust the duty cycle of the control pulse driving the first and second power switches 14 and 16, and as a result, regulating the output voltage VOUT1.
The primary inductor 24 is magnetically coupled to a secondary inductor 22 to form a transformer device 20. Some of the energy in the primary inductor 24 is transferred to the secondary inductor 22 when the second power switch 16 is xe2x80x9cONxe2x80x9d. The secondary inductor 22 provides a secondary output voltage proportional to the ratio of the number of turns in the secondary inductor 22 divided by the number of turns in the primary inductor 24. The secondary output voltage is added to the primary output voltage VOUT1 to provide an output voltage approximately equivalent to an auxiliary output voltage VOUT2. A diode 32 rectifies the output from secondary inductor 22 and the rectified signal is provided to the capacitor 28, which charges up to the auxiliary output voltage VOUT2. However, since the voltage drop across the diode varies with temperature and current, the auxiliary output voltage VOUT2 can easily vary 4-5%. This is unacceptable for the proper operation of many devices. Therefore, in some applications, the voltage provided at the capacitor 28 is increased and provided to a linear regulator 34. However, the linear regulator 34 has additional power and size requirements and decreases the efficiency of the switching regulator 10.
In some applications it is known to eliminate the linear regulator by providing a second feedback signal from the auxiliary output to a control circuit. The second feedback signal can be combined with the first feedback signal from the primary output through resistors so that only one feedback signal is provided to the control circuit. However, in this instance, the control circuit regulates the weighed average of both output voltages and may not provide adequate regulation for both outputs in some loading conditions.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention relates to circuits and a method for providing regulated output voltages from an unregulated input voltage. A switching power supply or switching regulator (e.g., a synchronous buck converter) is provided with a control circuit (e.g., pulse width modulator) that controls an input signal to a first and second switch coupled in series to an input voltage (e.g., unregulated). A first inductor or winding is coupled to a node between the first and the second switch. A primary output voltage is provided at the first inductor or winding during switching of the first and second switches. A second inductor or winding is magnetically coupled to the first inductor. The second inductor or winding provides a secondary output voltage based on the turns ratio of the second winding divided by the first winding.
An auxiliary switch is coupled to the second winding to rectify an auxiliary output voltage provided at another end of the auxiliary switch. A switch input control line is coupled between the node and the auxiliary switch to provide switching control to the auxiliary switch, such that both the auxiliary switch and the second switch are closed or on at the same time. The voltage drop of the auxiliary switch is matched with the voltage drop of second switch when both switches are closed or on, so that the two voltage drops across the switches substantially cancel one another out. Therefore, feedback from the auxiliary output voltage can be eliminated.
In one aspect of the invention, the first and second switches are N-type MOSFET devices and the auxiliary switch is a P-type MOSFET device. The gate of the P-type MOSFET device can be coupled to a node between the first and second switches. The matching of voltage drops can be accomplished by determining an xe2x80x9cONxe2x80x9d state resistance of the auxiliary switch based on the xe2x80x9cONxe2x80x9d state resistance of the second switch, the maximum load currents for both the primary and auxiliary output voltages and the turns ratio of the windings. Alternatively, the xe2x80x9cONxe2x80x9d state resistance of the auxiliary switch based on the xe2x80x9cONxe2x80x9d state resistance of the second switch can be selected for other loads (e.g. nominal) based on the particular application being employed.
The following description and the annexed drawings set forth certain illustrative aspects of the invention. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.